1. Field of the Invention
The present invention relates to ink jet printing systems and, more particularly, to an ink jet system having a preheating system which preheats ink during non-printing periods so as to prevent image density unevenness and to reduce the amount of energy to eject ink during printing periods.
2. Description of the Related Art
Ink jet printing systems have recently become very popular since they are both inexpensive and achieve high-quality color image printing. Conventionally, ink jet printers include interchangeable ink cartridges, integrating an ink tank for storing ink/dye and at least one print head for converting an electrical signal received from an external source, such as a personal computer, via the printer, into thermal energy- thereby discharging an ink droplet.
In such ink jet printers which employ print heads which utilize thermal energy for ink discharge, a known requirement for attaining high image quality is to control the temperature of both the print head and ink/dye which is discharged from the print head. This is because, if the temperature of the ink varies, the amount of ink discharged will vary depending on the temperature of both the print head and ink. As a result, unevenness in image density will appear in the printed image.
Conventionally, there are two methods of heating ink in order to eject ink in an ink jet printing system. The first method ejects ink droplets by performing thin-film boiling of the ink using heating elements at each print head nozzle. In this method of ejecting ink, the ink, which is either at room temperature or the environmental temperature within the printer, is moved from the ink tank directly to the nozzles of the print head where heat is applied in order to eject an ink droplet.
The second method ejects ink by raising the temperature of the ink from room or environmental temperature to a predetermined temperature while it is travelling to the print head and then performing thin film boiling of ink at each nozzle. In the second method, because the ink must be preheated first to the predetermined temperature, there is a short delay before printing can begin.
While both methods of heating ink to eject ink droplets are still used, the first method tends to produce droplets of varying sizes due to heat variations caused by non-uniformed heating of cool ink as it enters each nozzle. As a result, image density unevenness may occur. On the other hand, while the second method decreases non-uniformed heating of ink, due to preheating ink before it reaches each nozzle, the second method suffers from time delays which are required to preheat ink before printing.
Further attempts to those mentioned above have been made to reduce temperature dependent unevenness in image density and printing delays by measuring the temperature of the print head by using a temperature sensor or by estimating the temperature of the print head through record data and lookup tables. Other attempts have been made to control both the temperature of the print head and the ink by providing the print head with a heat generating member for heating the ink as it enters the print head. For example, by using a feed-back temperature control system by providing a temperature sensor in the print head.
In addition to the temperature sensor provided on the recording head, there may also be employed a highly precise temperature sensor, such as a thermistor, provided on the control circuit board in the main body of the printer. Such a configuration is used to detect the ambient temperature in the printer and to estimate the temperature of the print head and the ink in the ink tank by calculating from the variation in ambient temperature, the energy released through ink discharge and the energy dissipated to the external atmosphere.
However, even using the above temperature detection feed back, image density unevenness and time delays still occur because the ink/dye cannot be heated to the appropriate level before discharge. For example, if the ink/dye is not heated to the appropriate level at the expected time of discharge, the print head may misfire or the size of the ink droplet to be discharged, which is directly influenced by the amount of heat applied to the ink, may not be large enough for the appropriate image density thereby creating image density unevenness. As a result, image quality is affected as the ink and the print head heat up or down. Therefore, it is desirable to be able to provide an ink jet system which is capable of high quality image printing by controlling the heat of ink before the ink reaches the nozzles of the print head. Moreover, if the ink is maintained at a predetermined temperature prior to ejection, the amount of energy used to heat each nozzle heating element in the print head can be reduced.